Leafy greens are likely the number one food you can eat to regularly help improve your health. They're filled with fiber along with crucial vitamins, minerals, and plant-based phytochemicals that may help protect you from almost every disease known. Immune cells play an essential role in protecting intestinal health and could be boosted by consuming leafy greens, say researchers.

You can always count on leafy greens for some unsurpassed health benefits, if for no other reason than their exceptional nutrient richness.

Phytochemicals may be as important as any single nutrient in supplemental form. Phytochemicals are some of the most biologically active substances found on Earth. They give fruits, vegetables, legumes, and grains their rich colors, flavors, and aromas. But phytochemicals also detoxify the body by neutralizing free radicals, inhibiting enzymes that activate carcinogens, and most of all boosting immunity.

The new study in the Nature Immunology found that dietary factors, and in particular consumption of cruciferous leafy greens, control the activity of vital immune cells through the activation of a particular gene known as T-bet.

These immune cells, known as innate lymphoid cells (ILCs), play a vital role in protecting the body from infection by 'bad' pathogenic bacteria in our gut - and have also been suggested to play an important role in controlling food allergies, inflammatory diseases, obesity, and even bowel cancers, say the researchers writing Nature Immunology.

"In this study, we discovered that T-bet is the key gene that instructs precursor cells to develop into ILCs, which it does in response to signals in the food we eat and to bacteria in the gut," Dr Gabrielle Belz from the Walter and Eliza Hall Institute, Australia.

"ILCs are essential for immune surveillance of the digestive system and this is the first time that we have identified a gene responsible for the production of ILCs."

Belz said that the proteins in cruciferous vegetables are known to interact with a cell surface receptor that switches on T-bet -- and might therefore play a role in producing these critical immune cells.

"Proteins in these leafy greens could be part of the same signalling pathway that is used by T-bet to produce ILCs," she said.

"We are very interested in looking at how the products of these vegetables are able to talk to T-bet to make ILCs, which will give us more insight into how the food we eat influences our immune system and gut bacteria."

Delicate Balance

Belz and her team noted that ILCs are essential for maintaining the delicate balance between tolerance, immunity and inflammation -- by producing a hormone called interleukin-22 (IL-22), which can protect the body from invading bacteria.

"We are just starting to understand how important these immune cells are in regulating allergy and inflammation, and the implications for bowel cancer and other gastrointestinal disorders such as Crohn's disease," she said.

"Our research shows that, without the gene T-bet, the body is more susceptible to bacterial infections that enter through the digestive system," Belz added. "This suggests that boosting ILCs in the gut may aid in the treatment of these bacterial infections."

Jill Nussinow, MS, RD, a culinary educator in Northern California weighs in on the country's most widely-eaten greens from most nutritious to least:

Kale: This nutrition powerhouse "offers everything you want in a leafy green," says Nussinow, who gave it her first-place ranking. It's an excellent source of vitamins A C, and K, has a good amount of calcium for a vegetable, and also supplies folate and potassium. Kale's ruffle-edged leaves may range in color from cream to purple to black depending on the variety.

Before cooking with kale, collards, turnips, and chard, Nussinow recommends swishing the greens in a water-filled sink, draining the sink, then repeating this rinse until the leaves are dirt-free. Her favorite cooking method for these four greens is to rub the leaves in olive oil or tahini (sesame paste) and cook them for five minutes with garlic, olive oil, and broth.

Collards: Used in Southern-style cooking, collard greens are similar in nutrition to kale. But they have a heartier and chewier texture and a stronger cabbage-like taste. "Collards are an under-appreciated vegetable and most people don't know about them," suggests Nussinow. She says they're also popular with the raw food movement because the wide leaves are used as a wrapper instead of tortillas or bread. Down South, collards are typically slow cooked with either a ham hock or smoked turkey leg. A half cup has 25 calories.

Turnip greens: "If you buy turnips with the tops on, you get two vegetables in one," Nussinow tells WebMD. Turnip leaves are another Southern favorite traditionally made with pork. More tender than other greens and needing less cooking, this sharp-flavored leaf is low in calories yet loaded with vitamins A,C, and K as well as calcium.

Swiss chard: With red stems, stalks, and veins on its leaves, Swiss chard has a beet-like taste and soft texture that's perfect for sauteeing. Both Swiss chard and spinach contain oxalates, which are slightly reduced by cooking and can bind to calcium, a concern for people prone to kidney stones. Chard contains 15 calories in one-half cup and is a good source of vitamins A and C. Nussinow likes to make a sweet-and-sour chard by adding raisins and vinegar to the cooked greens.

Spinach: Popeye's favorite vegetable has 20 calories per serving, plus it's packed with vitamins A and C, as well as folate. And because heat reduces the green's oxalate content, freeing up its dietary calcium, "cooked spinach gives you more nutrition than raw," says Nussinow. Spinach leaves can be cooked quickly in the water that remains on them after rinsing, or they can be eaten raw in salads. Bags of frozen chopped spinach are more convenient to use than block kinds, and this mild-flavored vegetable can be added to soups, pasta dishes, and casseroles.

Mustard greens: Another Southern green with a similar nutrition profile to turnip leaves and collards, mustard greens have scalloped edges and come in red and green varieties. They have a peppery taste and give off a mustardy smell during cooking. Their spiciness can be toned down by adding an acid, such as vinegar or lemon juice, toward the end of cooking, suggests Nussinow. Cooked mustard greens have 10 calories in one-half cup.

Broccoli: With 25 calories a serving, broccoli is rich in vitamin C and is also a good source of vitamin A, potassium, and folate. Americans eat about 6 pounds of it a year. Its stalks and florets add both crunch and color to stir-fries. While some kids may call this veggie "trees," they often like it best raw or steamed with a yogurt-based dip. Nussinow mixes fresh broccoli into her pasta during the last three minutes of cooking so both are ready at the same time.

Red and Green Leaf and Romaine Lettuce: A familiar sight in salad bowls, these lettuces are high in vitamin A and offer some folate. Leaf lettuces have a softer texture than romaine, a crunchy variety used in Caesar salads. Fans of Iceberg lettuce may go for romaine, a crispy green that's better for you. Nussinow points out "the darker the lettuce leaf, the more nutrition it has," making red leaf slightly healthier than green. If you don't drown lettuce in a creamy dressing, one cup contains 10 calories.

Cabbage: Although paler in color than other leafy greens, this cruciferous vegetable is a great source of cancer-fighting compounds and vitamin C. Nussinow considers thisversatile green "the workhorse of the kitchen." Available in red and green varieties, cabbage can be cooked, added raw to salads or stir fries, shredded into a slaw, or made into sauerkraut. It's also a staple of St. Patrick's Day boiled suppers and can give off a strong smell when cooking. One-half cup cooked has 15 calories.

Iceberg Lettuce: This bland-tasting head lettuce is mostly water. But it's the country's most popular leafy green and each of us eats about 17 pounds of iceberg a year. While tops in consumption, it's last on our list for its health benefits. "It's not devoid of all nutrition, but it's pretty close," Nussinow stated.

Karen Foster is a holistic nutritionist, avid blogger, with five kids and an active lifestyle that keeps her in pursuit of the healthiest path towards a life of balance.